Work pieces for high temperature operation and method of making them



Sept. 1, 1959 E. FITZER WORK PIECES FOR HIGH TEMPERATURE OPERATION AND METHOD OF MAKING THEM Filed Aug. 12, 1955 1N VENTOR [We firzm ATTORNEY United States Patent WORK PIECES FOR HIGH OPER- ATION AND METHOD OF MAKING THEM This inventionis concerned with work pieces for high temperature operation, for example, heating rods for elec trical resistance furnaces, made of molybdenum-silicon alloys or comprising as a tinderproof component molybdenum silicide at least on the surface thereof which comes in contact with the oxidizing atmosphere, the parts of said work pieces which are exposed to lower working temperatures lying between about 300 C. and about 700 C., for'e'xampl'e, the terminal ends of heating rods, containing no molybdenum silicide coming in contact with oxygen gas, and with a method-of making such work pieces. 1

Silicon combinations of the transition metals of the 4th to 6th groups of the periodic system of elements, above all molybdenum silicide have been variously proposed for work pieces exposed to high and highest temperatures in oxidizing atmospheres. Such disilicide may thus be used in combination with silicon or hard materials or oxides as a sinter material or as a protective layer or coating upon molybdenum parts.

The baffling oxidation resistance up to about 1700 C. is believed traceable to the formation of a SiO -containing glasslike covering coating incident to annealing in air. This protective effect is, for example, said to make it possible to subject work pieces made of powdered materials, after a presintering in a reducing atmosphere, to high sintering in an oxidizing atmosphere, such high sintering being, for example, applied to rodlike parts by direct resistance heating thereof.

It has now been discovered that the formation of a tinderproof coating occurs only when the oxidation of the molybdenum silicide takes place at a temperature above about 700 C. The time until such coating forms depends upon the temperature. At about 750 C., the coating is formed after to 50 hours; at about 1000" C. already after about 5 to 10 hours; at 1400 C. after a few minutes. At between 300 C. and 700 C., especially between 500 C. and 650 C. the molybdenum silicide is surprisingly quickly destroyed due to the effect of oxygen. Sinter bodies have thus been destroyed within a few hours at 550" C. in an oxygen stream to complete disintegration to a grayish-green powder. The disintegration apparently proceeds from the particle borders by splitting off of Mosi -metallic particles which are enveloped by oxides. The disintegration is not affected by higher silicon content despite the fact that such condition improves the tinderproof quality of the molybdenum silicide at high and highest temperatures. This unexpected disintegration of the molybdenum silicide within the named temperature range occurs in similar manner in the case of bodies highly sintered in a hydrogen atmosphere or in an inert gas atmosphere. The consequence is that work pieces subjected to variable operating temperatures, for example, heating rods, suffer quick deterioration of the portions thereof which are disposed within the temperature ranges or zones lying between 300 C. to 700 C. It has for example been observed that a glasslike perfect appearing coating was formed after a short time, within 2,902,392 Patented Sept. 1, 1959 the high temperature zone, that is, at between 1400 C. and 1700 C., while the temperature zone between 300 C; and 700 C. suffered an increased attack after about 20 to 50 hours, with the result that the heating rods ruptured at the corresponding areas due to reduced crosssection' andoverheating at such areas.

The object of theinvention is accordingly to produce work pieces for high temperature operation, made of molybdenum silicon alloys or containing molybdenum silicide as'a tinderproof-component at the surface thereof which comes in contact with the oxidizing atmosphere, which work pieces are at certain areas thereof subjected to operating temperatures on the order of about 300 C. to about 700 C. In accordance with the invention, the'MoSi -surfaces must at such temperatures not come in contact with gaseous oxygen orwith an atmosphere containing gaseous oxygen.

In accordance with the invention, the MoSi -surfaces may be protected within the critical temperature range by coating them with a gastight protective coating consisting at least in part of oxides. A very simple and effective protection resides in the formation, upon MoSi work pieces, of a glasslike SiO -containing coating. Such coating maybe formed by annealing in knownmanner the parts of the Work pieces which will be subjected to temperatures between about 300 and about 700 C., in an oxygen-containing atmosphere, at temperatures lying above about 1300 C., preferably above about 1350" C.

The covering of the MoSi surfaces with metallic protective coatings produced thereon in known manner and by known means has likewise been found effective. Good results were for example obtained with electrolytically applied chromium coatings and with titanium silicide coatings sintered upon work pieces.

The accompanying drawing illustrates an example of a protective metallic coating on the MoSi surface of a work piece at the area thereof which lies within the critical temperature zone.

In the illustrated example, there is assumed a heating rod for electrical resistance furnaces of which is shown only a portion of the heater part and one terminal. The heating rod comprises the heater portion 1 and the terminal portion 3. The heater portion 1 contains at least on the surface thereof molybdenum disilicide and is provided with a glasslike protective coating 2, such coating being automatically formed upon the heater portion in an oxidizing atmosphere at the high operating temperature of between 1600 C. to about 1700 C. and protecting the heater portion 1 against further oxidizing attacks. At its terminal end, the heating rod is provided with an electrolytically applied chromium coating 5 upon which is sintered the terminal body 3 made of titanium silicide. The part 4 of the heating rod which contains molybdenum disilicide and which is exposed to operating temperatures on the order of about 300 C. to about 700 C. is thus protected against the ingress of air and oxidizing deterioration of the molybdenum silicide is along this critical area prevented. The curve 6 illustrates the temperatures occurring along the surface of the heating rod during the operation thereof in a resistance furnace.

Another way of forming such high temperatureproof silicide work pieces with zones or areas which remain even at lower temperatures unaffected by oxygen resides in combining the high temperature silicide part with a metallic part that is oxidationproof at lower temperature and is welded thereto. For example, there may be provided a molybdenum silicide heating conductor for working temperatures up to about 1700 C. having welded thereto at the opposite ends, which will be exposed to temperatures from about 500 C. to about 700 C., metallic parts made of silver and about 5% silicon. A

3 ferrite chromium steel may form the continuation to the cold copper terminal portion.

Changes may be made within the scope and spirit of the appended claims.

'l'claim:

1. For use in an-electn'c resistance furnacepa heating rod having a heating portionmadeof molybdenum =disilicide-Which is subjected in an oxidizing atmosphere to relatively high operatingtemperatureslyingbetween about 1000 C. and '1700 C., --a coating --consisting--substantially of silicon dioxide forming upon said heating portion incident to operative' heating thereof, such coating protecting said heating portion against destructivebxidation, andhaving a terminal (portion Which -issubjected to lower operating temperatures lyingbetween about 300 C. and about 700 C., said terminal 130111011 extending from said heating portion contiguous therewith and containing at least upon the surface thereof oxidation-proof material selected from the class of' materials consisting of chromium, titanium disilieideand -silver=silicon alloy to protect'said terminal portion against destructive oxidation in the presence of said-lower-temperatures.

-2. A structure according-to claim -1, wherein said oxidation-proof material is acoati-ng of titanium'disilici'de carried by saidterminalportion.

3. A structure according to claim 1, whereinsaidterminal portion consists of a body made of-t-itanium disilicide which is Welded to said heating-portion.

4. A structure according to claim 1, "wherein said terminal portion is a body made of silver silicon alloy containing about 5% silicon and welded to said heating portion.

5. A structure according to claim 1, wherein said oxidation-proof material is :'a coating of chromium carried by said terminal portion and having a body of titanium silicide sinte'red thereto.

References Cited in the file of this patent OTHER REFERENCES Harwood: Product Engineering, 1953, Annual Hand- Schwarzkopf et a1.: Refractory I-lard Metals, The

-MacMilla'n Co., 1953.

--Leadbeater et al.: Metal Treatment and Drop Forging, July 1954. I

Leadbeater et-al.: Metal Treatment and Drop Forging, August 1954. 

1. FOR USE IN AN ELECTRIC RESISTANCE FURNACE, A HEATING ROD HAVING A HEATING PORTION MADE OF MOLYBDENUM DISILICIDE WHICH IS SUBJECTED IN AN OXIDIZING ATMOSPHERE TO RELATIVELY HIGH OPERATING TEMPERATURES LYING BETWEEN ABOUT 1000* C. AND 1700* C., A COATING CONSISTING SUBSTANTIALLY OF SILICON DIOXIDE FORMING UPON SAID HEATING PORTION INCIDENT TO OPERATIVE HEATING THEREOF, SUCH COATING PROTECTING SAID HEATING PORTION AGAINST DESTRUCTIVE OXIDATION, AND HAVING A TERMINAL PORTION WHICH IS SUBJECTED TO LOWER OPERATING TEMPERATURES LYING BETWEEN ABOUT 300* C. AND ABOUT 700* C., SAID TERMINAL PORTION EXTENDING FROM SAID HEATING PORTION CONTIGUOUS THEREWITH AND CONTAINING AT LEAST UPON THE SURFACE THEREOF OXIDATION-PROOF MATERIAL SELECTED FROM THE CLASS OF MATERIALS CONSISTING OF CHRONIUM, TITANIUM DISILICIDE AND SILVER-SILICON ALLOY TO PROTECT SAID TERMINAL PORTION AGAINST DESTRUCTIVE OXIDATION IN THE PRESENCE OF SAID LOWER TEMPERATURES. 